Newcastle disease is a highly contagious viral disease affecting all species of birds. The disease can vary from an asymptomatic infection to a highly fatal disease, depending on the virus strain and the host species. Newcastle disease has a worldwide distribution and is a major threat to the poultry industries of all countries. Based on the severity of the disease produced in chickens, Newcastle disease virus (NDV) strains are grouped into three main pathotypes: lentogenic (strains that do not usually cause disease in adult chickens), mesogenic (strains of intermediate virulence) and velogenic (strains that cause high mortality).
NDV is a member of the genus Rubulavirus in the family Paramyxoviridae. The genome of NDV is a non-segmented, single-stranded, negative-sense RNA of 15186 nucleotides (Krishnamurthy & Samal, 1998; Phillips et el., 1998; de Leeuw & Peeters, 1999). The genomic RNA contains six genes that encode the following proteins in the order of: the nucleocapsid protein (NP), phosphoprotein (P), matrix protein (M), fusion protein (F), haemagglutinin-neuraminidase (HN) and large polymerase protein (L). Two additional proteins, V and W, of unknown function are produced by RNA editing during P gene transcription (Steward et al., 1993). A schematic diagram of the genetic map of NDV genomic RNA is shown in FIG. 1.
Three proteins, i.e. NP, P and L proteins, constitute the nucleocapsid. The genomic RNA is tightly bound by the NP protein and together with the P and L proteins form the functional nucleocapsid within which resides the viral transcriptive and replicative activities. The F and HN proteins form the external envelope spikes, where the HN glycoprotein is responsible for attachment of the virus to host cell receptors and the F glycoprotein mediates fusion of the viral envelope with the host cell plasma membrane thereby enabling penetration of the viral genome into the cytoplasm of the host cell. The HN and F proteins are the main targets for the immune response. The M protein forms the inner layer of the virion.
NDV follows the general scheme of transcription and replication of other non-segmented negative-strand RNA viruses. The polymerase enters the genome at a promoter in the 3′ extragenic leader region and proceeds along the entire length by a sequential stop-start mechanism during which the polymerase remains template bound and is guided by short consensus gene start (GS) and gene end (GE) signals. This generates a free leader RNA and six non-overlapping subgenomic mRNAs. The abundance of the various mRNAs decreases with increasing gene distance from the promoter. The genes are separated by short intergenic regions (1-47 nucleotides) which are not copied into the individual mRNAs. RNA replication occurs when the polymerase somehow switches to a read-through mode in which the transcription signals are ignored. This produces a complete encapsulated positive-sense replicative intermediate which serves as the template for progeny genomes.
Reverse-genetic techniques have been reported to recover negative-sense viruses from cloned cDNA (Conzelmann, 1996). For NDV, reverse-genetic technology is currently available for avirulent strain LaSota (Römer-Oberdörfer et al., 1999; Peeters et al., 1999).